(a) Technical Field
The present invention relates to a photocatalyst using a semiconductor-carbon nanomaterial core-shell composite quantum dot and a method for preparing the same, more particularly to a microparticle in which a semiconductor-carbon nanomaterial core-shell composite quantum dot is self-assembled using 4-aminophenol, capable of improving photoelectrochemical response and solar conversion efficiency when used as a photoelectrode of a photoelectrochemical device, a photoelectrochemical device using the same and a method for preparing the same.
(b) Background Art
Various methods and materials are being developed to solve environmental problems. Among them, a photocatalyst is advantageous in that it does not cause incidental pollution because it decomposes organic pollutants by using sunlight. The photocatalyst refers to “a material which facilitates a chemical reaction occurring in the presence of light, without being consumed”. It accelerates the reaction using the light as an energy source. Typically, semiconductors, metal oxides or sulfur compounds are used as the photocatalyst. It is known that the photocatalyst can decompose various non-biodegradable materials that cannot be degraded by microorganisms. Materials exhibiting such photocatalytic effect include ZnO, WO3, SnO2, ZrO2, TiO2, etc.
Among the materials used as photocatalysts and photoelectrodes of photoelectrochemical devices, those exhibiting superior photocatalytic activity but very low solar energy conversion efficiency due to inability to absorb sunlight in the visible region owing to large bandgap energy include zinc oxide, titanium oxide, etc. However, because of their large bandgap energy, they cannot absorb the light in the visible region and can only absorb light in the UV region with wavelengths 400 nm or shorter. Accordingly, there is a limitation in improving solar energy conversion efficiency and the device performance is very poor.
To solve this problem, a method of growing a ZnO nanowire on an FTO (F-doped SnO2)/glass substrate and then functionalizing the ZnO nanowire with a graphene quantum dot to improve the absorption efficiency of sunlight in the visible region was proposed [C. X. Guo et al., Graphene Quantum Dots as a Green Sensitizer to Functionalize ZnO Nanowire Arrays on F-Doped SnO2 Glass for Enhanced Photoelectrochemical Water Splitting, Adv. Energy Mater., 3, 997 (2013)]. Similarly, a method of growing a ZnO nanowire as a core on a substrate and then forming a CoNi shell to give a double-layered composite exhibiting improved photocatalytic performance was reported [M. Shao et al., Hierarchical Nanowire Arrays Based on ZnO Core-Layered Double Hydroxide Shell for Largely Enhanced Photoelectrochemical Water Splitting. Adv. Funct. Mater. 24, 580 (2013)].
In addition, there is a method of improving energy conversion efficiency by introducing a semiconductor material capable of absorbing sunlight in the visible region together with zinc oxide or titanium oxide [A. Kudo and Y. Miseki, Heterogeneous photocatalyst materials for water splitting. Chem. Soc. Rev. 38, 253 (2009)]. Additionally there is a method of improving light harvesting performance by introducing a semiconductor material capable of absorbing sunlight in visible region, but it has no choice but to be assisted by a photovoltaic device using tandem system in order to supply the required energy for water splitting [J. K. Kim et al., Synthesis of transparent mesoporous tungsten trioxide films with enhanced photoelectrochemical response: application to unassisted solar water splitting Energy Environ. Sci. 4, 1465 (2011)].
However, because these techniques apply two or more different materials to a device, the charge carriers generated from the sunlight absorption tend to recombine and be lost during the charge transport or charge transfer. As a result, it is difficult to achieve maximized energy conversion efficiency.
Meanwhile, as a method of improving photocatalytic activity by improving charge transport under limited sunlight absorption, a method of binding a composite material on a micrometer-scale graphene sheet is known [Y. Bu, Z. Chen, W. Li Dramatically enhanced photocatalytic properties of Ag-modified graphene-ZnO quasi-shell-core heterojunction composite material. RSC Advances DOI: 10.1039/c3ra44047h].
However, the improvement of charge transport is limited since the composite material contacts with an electrolyte over a large area because the graphene does not entirely surround the material. In addition, because many nanoparticles are attached to the graphene sheet, it is difficult to be prepared as a thin film and is not suitable for application to photoelectrochemical devices.
Called the next-generation dream material, graphene having a two-dimensional structure in which carbon atoms are arranged as a single layer exhibits excellent thermal conductivity, electron mobility and flexibility although it was less studied as compared to other nanocarbon materials such as carbon nanotube (CNT), fullerene, graphite, etc. For this reason, intense researches are focused on graphene.
In particular, use of materials exhibiting superior electrical properties, such as graphene or fullerene, can lead to significant improvement in charge transport and greatly improved durability by preventing photocorrosion. Also, graphene can be used as an electrode material for secondary batteries, supercapacitors, solar cells, etc. and, particularly, is useful as an additive for a charge transport layer and an active layer of a solar cell because of high charge mobility. In addition, since graphene has its own specific electrical properties with both metallic and semi-conductive properties and has a planar structure with a large specific surface area, it is recently drawing attention for use in transparent electrodes, electrochemical devices, supporting media for the catalysts, etc. Graphene exhibits different properties depending on the number of layers forming it. A multi-layer graphene is reported to exhibit semiconductive properties and help formation of nanosized crystals when bound to metal or metal oxide. Thus, it is studied a lot for use as a supporting media for the photocatalysts. However, since preparation of pure graphene is limited in terms of production cost and efficiency, use of graphene itself as a supporting media for the catalysts is impractical.
Accordingly, there is a need of the development of a new photocatalyst material which exhibits maximized photocatalytic performance and performs in the visible region in the field of environmental technology and energy technology.
Korean Patent Publication No. 2013-0113770 proposes a hybrid photocatalyst nanoparticle having improved photoactivity and a method for preparing the same and Korean Patent Publication No. 2013-0070327 describes a method of manufacturing a graphene sheet which is combined with a titanium dioxide nanorod and application thereof as a photocatalyst responding to visible light.
However, these techniques are also limited in maximizing photocatalytic performance and application for the visible region.